An EL device, which can be largely classified into an inorganic EL device and an organic EL device, has high visibility due to its self-emission, and is excellent in impact resistance and easy to handle since it is a completely solid device. For these reasons, research, development and practical utilization thereof as a pixel for graphic display, a pixel for a television image display device or a surface light source are under way.
An organic EL device has a laminated structure in which a light-emitting layer formed of a fluorescent organic solid such as anthracene, etc., and a hole-injecting layer formed of triphenylamine, etc., are provided, a light-emitting layer and an electron-injecting layer formed of a perylene derivative, etc., are provided, or a hole-injecting layer, a light-emitting layer and an electron-injecting layer are provided, between two electrodes (the electrode on the light-emitting side is a transparent electrode). This laminated structure is generally formed on a substrate.
Such an organic EL device utilizes light emission caused when electrons injected into a light-emitting layer and holes are recombined. Therefore, an organic EL device has advantages that it can be actuated at a low voltage, e.g., 4.5 V and response is fast by decreasing the thickness of the light-emitting layer, and it also has advantages that it gives a high-brightness EL device since the brightness is in proportion to an injected electric current. Further, by changing the kinds of fluorescent organic solids of which the light-emitting layer is to be formed, light emission is obtained in all the colors in the visible regions of blue, green, yellow and red. Since an organic EL device has these advantages, particularly an advantage that it can be actuated at a low voltage, studies thereof for practical use are under way at present.
Meanwhile, a fluorescent organic solid used as a material for forming the light-emitting layer of an organic EL device is susceptible to water, oxygen, etc. Further, an electrode (to be sometimes referred to as "opposite electrode" hereinafter), which is formed on a light-emitting layer directly or through a hole-injecting layer or an electron-injecting layer, is liable to deteriorate in properties due to oxidation. As a result, when a conventional EL device is actuated in the atmosphere, its life as a device is short. Therefore, in order to obtain a practically usable organic EL device, it is required to increase the life thereof by protecting the device so that water, oxygen, etc., do not infiltrate the light-emitting layer and that the opposite electrode does not undergo oxidation.
However, no effective protection method has been developed for the organic EL device. For example, when a method of protecting (sealing) an inorganic EL device, i.e., a method in which a back-surface glass plate is provided outside a back electrode (opposite electrode) and a silicone oil is placed between the back electrode and the back-surface glass and sealed, is applied to the organic EL device, the silicone oil infiltrates the light-emitting layer through the opposite electrode or through the opposite electrode and the hole-injecting layer or the electron-injecting layer, and the light-emitting layer is altered due to the silicone oil. Therefore, the organic EL device immensely deteriorates in light emission properties, or no longer emits light. When a resin coating layer generally used for mechanical protection (the solvent for the coating solution is a cyclic ether-containing solvent such as tetrahydrofuran, etc., a halogen-containing solvent such as chloroform, dichloromethane, etc., or an aromatic hydrocarbon-based solvent such as benzene, toluene, xylene, etc.) is applied to tile protection of the organic EL device, the solvent used in the resin coating solution infiltrates the light-emitting layer like the above silicone oil. Therefore, the organic EL device greatly deteriorates in light emission properties, or does not at all emit light.
It is therefore an object of the present invention to provide an organic EL device which can be produced so that it structurally has a long life.